Patent Application
20150017043
This disclosure relates to powder metal formulations including solid lubricants and further relates to powder metal parts, such as scroll compressors, made using these powder metal formulations.
Scroll compressors are typically used for compressing gases or a refrigerant. In such a scroll compressor, two parts are situated so as to have interleaving and complimentary scroll portions. These scroll portions may be shaped as involutes, spirals, or other such curves.
During operation of the scroll compressor, one of the scroll portions is gyrated relative to the other scroll portion. This movement of the scroll portions relative to one another causes the points of contact between the two scroll portions to vary. These changing points of contact between the scrolls, when made over a continuous length, can result in the forced movement and the compression of gases and/or refrigerants between the two parts.
As noted above, the parts for a scroll compressor can have relatively complex geometries (i.e., may have an involute or a spiral shape) and can present fabrication challenges. Because powder metallurgy is well adapted to handle certain complex geometries and high part volumes, powder metal processes have been explored as one means to make the scroll portion of a scroll compressor or, more ambitiously, all of a scroll compressor.
Powder metal parts may be fabricated in the following manner. A powder metal starting material is compacted under pressure using a die and tool set to form the loose powder metal into a powder metal compact. This powder metal compact has a shape that is relatively close to, but slightly larger than, the shape of the final desired part. This powder metal compact is then sintered to cause the adjacent powder metal particles to diffuse into one another and to neck together thereby bonding the particles together. This sintering is typically done at just below the melting temperature of the powder metal material but, in some instances, a liquid phase may also be developed during sintering. In comparison to the initial powder metal compact, the sintered powder metal forms a much stronger sintered part that might be subjected to any of a number of finishing processes (e.g., machining, grinding, deburring, and so forth), reworking (e.g., forging or coining), or simply used as-sintered.
A powder metal formulation is disclosed which is particularly useful in the production of powder metal scroll compressors. This powder metal formulation includes a solid lubricant such as, for example, talc or boron nitride which is carried through the powder metal formation process such that the solid lubricant is part of the final powder metal scroll compressor. In another example, the solid lubricant is a nickel-coated graphite powder. In some forms, the solid lubricant is admixed with the other constituents of the powder metal material. These solid lubricants remain stable at the elevated processing temperatures employed during the sintering of the powder metal and so this solid lubricant remains in and, at least to some degree, available at the surface of the powder metal part after sintering.
Using this powder metal as a starting material, a scroll compressor can be made that includes a solid lubricant. Among other things, this solid lubricant helps promote smooth contact between the scrolls with reduced amounts of friction. The solid lubricant is also inert such that it does not present any concerns when it is used to compress, for example, a refrigerant.
According to one aspect, a powder metal scroll compressor is provided. The powder metal scroll compressor includes a hub and a scroll adjoined to one another in which a powder metal forms at least a portion of the powder metal scroll compressor including the scroll. The powder metal includes iron powder, carbon in an amount of less than 0.9% by weight of the powder metal, and a solid lubricant in the powder metal.
The iron powder and solid lubricant may be admixed with one another prior to compaction and sintering of the powder metal scroll compressor.
In one form, the solid lubricant may be 0.25% to 3.0% by weight of the powder metal and the powder metal may include only iron powder, carbon, the solid lubricant and be substantially free of other constituents.
However, in other forms, the powder metal may contain other constituents. For example, the powder metal may further include copper powder (which may be elemental copper powder) in an amount of less than 3.0% by weight of the powder metal. In this form, the iron powder, the copper powder, and the solid lubricant may be admixed with one another prior to compaction and sintering of the powder metal scroll compressor and the powder metal may include only iron powder, carbon, copper powder, and the solid lubricant and be substantially free of other constituents.
Various types of solid lubricants may be suitable for use in the powder metal. In order to provide the lubricating function in the final component, the solid lubricant should be capable of surviving the compaction and sintering process (for example, not burn off at sintering temperatures). Thus, one having ordinary skill in the art will appreciate that the solid lubricants being referred to are not typical lubricants, waxes, or binders that are conventionally used to help the compacted powder metal parts retain their shape or be ejected from the compaction tooling, as those conventional lubricants, waxes, or binders are consumed and lost during any initial burn off and/or sintering operations. Accordingly, many of the solid lubricants described herein remain inert and stable in an Fe—C or an Fe—Cu—C system through processing of temperatures up to 1080 degrees Centigrade, for example.
One solid lubricant that may be used is talc (Mg3Si4O10(OH)2). The talc may have a nominal 15 to 25 micron mean particle size (d50).
Another solid lubricant that may be used is hexagonal boron nitride (BN). The hexagonal boron nitride may have a nominal 5 to 30 micron mean particle size (d50).
The solid lubricant may be provided in the form of a nickel-coated graphite powder. In this instance, the carbon may be present in an amount of less than 0.9% by weight of the powder metal material, exclusive of the graphite of the nickel-coated graphite powder (as this graphite powder does not significantly contribute to the carbon content in the iron). A nickel coating of the nickel-coated graphite powder may substantially surround the graphite to protect the graphite during sintering of the powder metal scroll compressor and to prevent the graphite from combining with the iron powder. A nickel content of the nickel-coated graphite powder may be in a range of 55 to 80 wt % with the remainder being graphite. A total amount of graphite in the powder metal scroll compressor may be in the range of 0.5 to 5.0 wt %, or more narrowly 1.0 to 3.0 wt %, exclusive of the carbon in an amount of less than 0.9% by weight of the powder metal material. The nickel-coated graphite powder may have an average particle size of approximately 100 microns.
According to another aspect, a powder metal is provided, such as a powder metal that may be used for a powder metal scroll compressor of the type described above. The powder metal includes iron powder, carbon in an amount of less than 0.9% by weight of the powder metal material, and a solid lubricant. The iron powder and solid lubricant are admixed with one another.
In one form of the powder metal, the powder metal may include iron powder, carbon, the solid lubricant and be substantially free of other constituents.
In another form, the powder metal may further include a copper powder (such as an elemental copper powder) in an amount of less than 3.0% by weight of the powder metal. The iron powder, the copper powder, and the solid lubricant may be admixed with one another, and the powder metal may include iron powder, carbon, copper powder, and the solid lubricant and be substantially free of other constituents.
The solid lubricant may be 0.25% to 3.0% by weight of the powder metal. For the reasons identified above relating to the retention of the solid lubricant in the final part, the solid lubricant may remain inert and stable in an Fe—C or an Fe—Cu—C system through processing of temperatures up to 1080 degrees Centigrade.
In one form of the powder metal, the solid lubricant may be talc (Mg3Si4O10(OH)2). The talc may have a nominal 15 to 25 micron mean particle size (d50).
In another form, the solid lubricant may be hexagonal boron nitride (BN). The hexagonal boron nitride may have a nominal 5 to 30 micron mean particle size (d50).
In still another form, the solid lubricant may be a nickel-coated graphite powder and in which the carbon in an amount of less than 0.9% by weight of the powder metal material is exclusive of the graphite of the nickel-coated graphite powder.
A nickel coating of the nickel-coated graphite powder may substantially surround the graphite to protect the graphite during sintering of the powder metal scroll compressor and to prevent the graphite from combining with the iron powder. A nickel content of the nickel-coated graphite powder may be in a range of 55 to 80 wt % with the remainder being graphite. A total amount of graphite in the powder metal scroll compressor may be in the range of 0.5 to 5.0 wt %, or more narrowly 1.0 to 3.0 wt %, exclusive of the carbon in an amount of less than 0.9% by weight of the powder metal material. The nickel-coated graphite powder may be an average particle size of approximately 100 microns.
A part may be made using any of the powder metal formulations described herein by compacting and sintering the powder metal to form the part. The solid lubricant is retained throughout the process and is dispersed throughout the part including the surface of the part. It may be particularly advantageous when this surface of the part is a bearing surface such that the solid lubricant can serve as a lubricant on this surface.
These and still other advantages of the invention will be apparent from the detailed description and drawings. What follows is merely a description of some preferred embodiments of the present invention. To assess the full scope of the invention, the claims should be looked to as these preferred embodiments are not intended to be the only embodiments within the scope of the claims.
Referring first to
To be clear, in the instant disclosure, the powder metal formulation could be used to separately make one or more portions of the part or the entirety of the part using, for example, the methods described in PCT International Publication No. WO 2010/135232 and U.S. Patent Application Publication No. US 2012/0118104. However, the details of the structure and the processes used to fabricate the scroll compressor should not be so limited to only the structures and methods explicitly listed. Accordingly, the below described scroll compressor is intended to be illustrative, but not limiting.
The scroll compressor 100 is a powder metal part which is formed by compression along an axis of compaction A-A. The scroll compressor 100 includes a flange 102, a hub 104, and a scroll 106. The flange 102 has a top face 108 and a bottom face 110 which extend in a direction perpendicular to the axis A-A and which are essentially planar and parallel to one another. Two mounting slots 112 are formed around an outer periphery of the flange 102 for mounting the scroll compressor 100 to another item in a refrigeration assembly or the like.
The hub 104 axially extends from the top face 108 of the flange 102. The hub 104 is generally cylindrically-shaped and has a radially outward facing surface 114, a radially inward facing surface 116, and a top axial face 118. Both the radially outward facing surface 114 and the radially inward facing surface 116 may have a slight taper as they extend away from the top face 108 of the flange 102 towards the top axial face 118. By having a taper, the scroll compressor 100 can be more easily separated from the tool members during the ejection process.
The scroll 106 extends axially from the bottom face 110 of the flange 102. The scroll 106 is a spiraling wall that spirals relative to the axis A-A. As such, the scroll 106 includes an inner wall end 120 and an outer wall end 122 with a generally radially outward facing surface 124 and a generally radially inward facing surface 126 extending between the ends 120 and 122. These surfaces 124 and 126 run generally parallel to one another as they spiral away from the axis A-A, creating a spiraling wall of uniform thickness. A bottom axial face 128 of the scroll 106 is also spiral-shaped. Again, the generally radially outward facing surface 124 and the generally radially inward facing surface 126 may have a taper to ease the ejection process from the tool and die set during compaction of the powder metal.
In the form shown, the spiral is similar to an Archimedean spiral, meaning that if a radial line is drawn relative to the axis A-A, a channel 130 formed between the generally radially outward and inward facing surfaces 124 and 126 is also of substantially constant width regardless of the distance from the axis A-A. However, other involute geometries might be used and nothing should limit the scroll compressor geometry to that which is illustrated in
A part having this geometry could not be easily formed as a unitary powder metal compact by a conventional powder metal compaction process. Typically, top features, such as the hub 104 are formed by transferring powder metal within the die cavity by a powder transfer motion of the lower tool members. As the powder is transferred, the powder fill to final part ratio along the vertical columns of the part must be approximately 2:1 to provide a part that is relatively uniformly dense after the compaction process.
A comparison of a horizontal cross section through the hub 104 and a horizontal cross section through the scroll 106 would reveal that there are areas of powder metal in the hub 104 which are not found in the scroll 106 and areas of powder metal in the scroll 106 that are not found in the hub 104. Thus, conventional tool and die sets are incapable of performing a powder transfer motion that provides an acceptable powder fill to final part ratio over a component having this final geometry. Instead, the hub and scroll sections are conventionally separately compacted and then joined afterwards. However, PCT International Publication No. WO 2010/135232 and U.S. Patent Application Publication No. US 2012/0118104 describe ways of fabricating a unitary part. Parts made using both the conventional and improved methods are contemplated as being within the scope of this invention.
Turning now to the powder metal formulation, the powder metal used to make this powder metal scroll compressor includes iron powder (either elemental or prealloyed iron), carbon in an amount less than 0.9 wt % of the powder metal, and solid lubricant in an amount between 0.25 wt % and 3.0 wt % of the powder metal. Other elemental additions could also be included such as, for example, copper (Cu) and nickel (Ni). However, the powder metal formulation is relatively simplistic in that it does not require more than these listed constituents, but may include trace amounts of other elements that do not substantially affect the properties of the powder metal.
Mixing of the various constituent powders may be performed using conventional means such as v-blenders or double cone mixers. The various constituent powders can be admixed together along with pressing lubricants (e.g., lithium stearate, Licowax, etc.) in addition to the solid lubricant.
In one form, the solid lubricant for this powder metal formulation may be talc, which is also known as hydrated magnesium silicate and has the chemical formula of Mg3Si4O10(OH)2. When used as a solid lubricant, some amount of quartz impurity may exist within the talc. When talc is used as the solid lubricant, the talc may be preferably provided in a powder form having a nominal 15 to 25 micron mean particle size (d50).
In another form, the solid lubricant for this powder formulation may be a hexagonal boron nitride (BN). When used as a solid lubricant, the boron nitride may be preferably provided in a powder form having a nominal 5 to 30 micron mean particle size (d50).
Another approach to provide an in-place solid lubricant in a powder metal component is to use nickel-coated graphite powder as the solid lubricant. The nickel coating protects the graphite during sintering and prevents the graphite from combining with the iron powder. In the finished product, the coating protects and preserves the graphite until rupture of the nickel coating during use of the component (e.g., rupture due to wear on surfaces), to release the graphite lubricant. The nickel-coated graphite has nickel content ranging from 55 to 80 wt % with the remainder being graphite. The nickel-coated graphite powder may have an average particle size of approximately 100 microns. Graphite size may be coarse (Tyler mesh size −120/+230 at 88-98% or a range of 115 to 65 microns) to fine (Tyler mesh size −120/+270 at >85% and −270/+325<15% or a range of 115 to 43 microns) with both having a small amount of more coarse and finer particle sizes (<5 wt %). The solid lubricant is added to the mix to provide a graphite level of 0.5 wt % to 5 wt %, although a graphite range of 1 wt % to 3 wt % is believed to be typical for most applications. This graphite is exclusive of the carbon content in the powder metal which is used to alter the metallurgical properties of the iron powder.
Notably, this powder metal formulation incorporates the powder for the solid lubricant as an admixed constituent in the powder metal. The admixed solid lubricant is inert in Fe—C, Fe—Cu—C and other powder metal mixtures processed at temperatures around 1180 degrees Centigrade. This means that even after the powder metal has been compacted and sintered into a final part, all or a substantial portion of the solid lubricant remains present.
The solid lubricant assists in reducing frictional heat and spalling/galling in a system with reciprocal motion under a mechanical load such as that in which a scroll compressor is used (and, in particular, in the scroll section of the scroll compressor). The solid lubricant is resistant to adhesion and does not create significant resistance to motion of the scroll compressor. The inclusion of the solid lubricant may be best implemented in powder metal parts that have ferritic/pearlitic microstructures and its inclusion can also be used for improving machinability.
In one specific embodiment that is particularly advantageous for powder metal scroll compressors, the powder metal formulation can be formulated to have less than 0.9 wt % carbon, less than 3.0% copper, and between 0.25 to 3.0 wt % of solid lubricant with the remainder of the powder being elemental iron with no other substantial additions. Again, this is a relatively simple powder formula that does not contain a large number of alloying elements.
Powder metal formulations such as those described above can be prepared and then processed into a sintered powder metal part by compacting the powder into a powder metal compact and then sintering the powder. It is contemplated that such parts might be compacted as unitary bodies or might be formed from separately compacted components that are subsequently joined together to form a single final part. However, it is contemplated that any section of a part made from various joined sections may have the powder metal containing the solid lubricant in those sections in which the solid lubricant will be most desirable. For example, the scroll section of a scroll compressor may be made using the powder described above, while the other section to which the scroll section is joined may be made of a powder metal material that does not include the solid lubricant. Of course, nothing excludes both sections of a multi-portion component from being made of a powder containing the solid lubricant even if they are joined.
Overall, this process allows for conventional compaction processes in rigid dies and eliminates the subsequent infiltration of a solid lubricant into the porous sintered body after sintering.
It should be appreciated that various other modifications and variations to the preferred embodiments can be made within the spirit and scope of the invention. Therefore, the invention should not be limited to the described embodiments. To ascertain the full scope of the invention, the following claims should be referenced.
This claims the benefit of U.S. Provisional Patent Application No. 61/599,042 filed Feb. 15, 2012 and U.S. Provisional Patent Application No. 61/720,226 filed Oct. 30, 2012. The contents of both of these applications are incorporated by reference as if set forth in their entirety herein for all purposes.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US13/25576 | 2/11/2013 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61720226 | Oct 2012 | US | |
| 61599042 | Feb 2012 | US |
